U.S. patent number 5,413,986 [Application Number 07/691,057] was granted by the patent office on 1995-05-09 for method for production of thin oxide superconducting film and substrate for production of the film.
This patent grant is currently assigned to Kabushiki Kaisha Komatsu Seisakusho. Invention is credited to Kozo Nakamura.
United States Patent |
5,413,986 |
Nakamura |
May 9, 1995 |
Method for production of thin oxide superconducting film and
substrate for production of the film
Abstract
A method for producing a thin oxide superconducting film
possessing high crystallinity and excellent quality and a novel
single crystal as a substrate allowing easy formation of an
epitaxial film of high quality usable for the method are provided.
The method for the production of the thin oxide superconducting
film is characterized by using as a substrate a single crystal of
SrLaGaO.sub.4 which is a high-melting oxide and effecting epitaxial
growth of a thin oxide superconducting film on the substrate. The
single crystal used as a substrate is an oxide single crystal
possessing a crystal structure of the K.sub.2 NiF.sub.4 type and
having a composition of Sr.sub.1-X La.sub.1-Y Ga.sub.1-Z O.sub.4-W
(wherein X, Y, Z, and W fall in the following respective ranges;
-0.1<X<0.1, -0.1<Y<0.1, -0.1<Z<0.1, and
-0.4<W<0.4).
Inventors: |
Nakamura; Kozo (Hiratsuka,
JP) |
Assignee: |
Kabushiki Kaisha Komatsu
Seisakusho (Tokyo, JP)
|
Family
ID: |
18158074 |
Appl.
No.: |
07/691,057 |
Filed: |
June 21, 1991 |
PCT
Filed: |
December 25, 1989 |
PCT No.: |
PCT/JP89/01299 |
371
Date: |
June 21, 1991 |
102(e)
Date: |
June 21, 1991 |
PCT
Pub. No.: |
WO90/07591 |
PCT
Pub. Date: |
July 12, 1990 |
Foreign Application Priority Data
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|
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Dec 23, 1988 [JP] |
|
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63-323739 |
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Current U.S.
Class: |
505/476; 505/447;
505/473; 505/451; 505/238 |
Current CPC
Class: |
C30B
23/02 (20130101); C30B 29/22 (20130101); C30B
29/225 (20130101); H01L 39/2458 (20130101) |
Current International
Class: |
C30B
23/02 (20060101); C30B 025/18 () |
Field of
Search: |
;156/600,610,613,614
;505/1,729,730,238,239,330,447,973 ;117/90,89,95,105 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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|
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50-134200 |
|
Oct 1975 |
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JP |
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63-260896 |
|
Oct 1988 |
|
JP |
|
2267119 |
|
Oct 1990 |
|
JP |
|
4-2697 |
|
Jan 1992 |
|
JP |
|
WO904857 |
|
May 1990 |
|
WO |
|
Other References
Moriwaki et al., "Electrical Properties of Superconducting (LaSr)
CuOg And BaYCuO Thin Films," High Temperature Superconductors Ext.
abs, MRS Apr. 1987, pp. 85-87. .
Japanese Journal of Applied Physics, vol. 27, No. 7, pp.
L1293-L1296, "Formation of Bi-Sr-Ca-Cu-O Thin Films by a Laser
Sputtering Method", Masaki Kanai et al., Jul. 1988..
|
Primary Examiner: Kunemund; Robert
Attorney, Agent or Firm: Armstrong, Westerman, Hattori,
McLeland & Naughton
Claims
What is claimed is:
1. A method for the production of a thin oxide superconducting
film, comprising
providing a substrate of a single crystal comprising Sr.sub.1-X
La.sub.1-Y Ga.sub.1-Z O.sub.4-W, wherein X, Y, Z, and W fall in the
following respective ranges: -0.1<X<0.1, -0.1<Y<0.1,
-0.1<Z<0.1, -0.4<W<0.4 and
forming on the substrate by epitaxial growth a thin oxide
superconducting film possessing lattice constants a and b in the
range of 3.76 to 3.92 .ANG. or in the range of 5.32 to 5.54 .ANG.,
wherein said oxide superconducting film comprises an oxide of the
Bi--Sr--Ca--Cu--O type.
2. A method for the production of a thin oxide superconducting
film, comprising
providing a substrate of a single crystal comprising Sr.sub.1-X
La.sub.1-Y Ga.sub.1-Z O.sub.4-W, wherein X, Y, Z, and W fall in the
following respective ranges: -0.1<X<0.1, -0.1<Y<0.1,
-0.1<Z<0.1, -0.4<W<0.4 and
forming on the substrate by epitaxial growth a thin oxide
superconducting film possessing lattice constants a and b in the
range of 3.76 to 3.92 .ANG. or in the range of 5.32 to 5.54 .ANG.,
wherein said oxide superconducting film comprises an oxide of the
Tl--Ba--Ca--Cu--O type.
Description
FIELD OF THE INVENTION
This invention relates to a method for the production of a thin
oxide superconducting film and to a Sr--La--Ga type oxide single
crystal usable advantageously as a substrate for the production
mentioned above.
BACKGROUND OF THE INVENTION
The electronic devices utilizing the phenomenon of superconduction
find extensive utility in various applications as high-speed
switches, high-sensitivity detectors, and high-sensitivity
fluxmeters.
These superconducting devices are constructed with thin
superconducting films. Since the thin superconducting films have
considerably low critical superconducting temperatures (Tc) and,
therefore, require use of liquefied helium as a cooling agent, they
have encountered the problem of high cost, complexity of overall
system, and incapability of size reduction.
Thus, studies have been promoted on thin oxide superconducting
films possessing high critical superconducting temperatures. Thin
oxide superconducting films discovered in recent years have
critical superconducting temperatures exceeding 77.degree. K. and,
therefore, are capable of being operated by the use of inexpensive
liquefied nitrogen as a cooling agent.
For the production of thin oxide films of this class, the method
which comprises superposing a given thin film by the spattering
method or the vacuum evaporation method on a MgO single-crystal
substrate or a SrTiO.sub.3 single-crystal substrate heated in
advance to an elevated temperature has been used. It has been
proposed in recent years for the purpose of enhancing the epitaxial
growth of a thin oxide film to use as a substrate therefor an oxide
insulator possessing a lattice-matching property relative to a
given oxide superconductor and, at the same time, containing at
least one of the component elements of the superconductor (Japanese
Patent Application Disclosure SHO 63(1988)-236,794).
The conventional method for the production of a thin film by the
use of a MgO single crystal or a SrTiO.sub.3 single crystal as a
substrate does not easily produce an epitaxial film of high
quality. This fact has posed itself a serious problem for the
stabilization of critical superconducting temperature (Tc) and for
the improvement and stabilization of the critical superconducting
current (Jc).
To allow the growth of an excellent epitaxial film, the material
for the substrate must fulfil the following requirements, for
example:
(i) It should exhibit a highly satisfactory lattice-matching
property to a thin crystal film.
(ii) It should avoid deteriorating film quality due to mutual
diffusion of the film and the substrate during the epitaxial growth
of the film.
(iii) It should possess a high melting point exceeding at least
1,000.degree. C. to withstand the heating at an elevated
temperature.
(iv) It should be procurable in the form of a single crystal
possessing highly satisfactory crystallinity.
(v) It should be an insulator of electricity.
Incidentally, numerous oxides such as those of the InBa.sub.2
Cu.sub.2 O.sub.7-.delta. (.delta.=0 to 1, Ln=Yb, Er, Y, Ho, or Gd)
type, the Bi--Sr--Ca--Cu--O type, and the Tl--Ba--Ca--Cu--O type,
for example, have been reported as high-temperature oxide
superconductors. The lattice constants, a and b, of these oxide
superconductors are invariably in the range of 3.76 to 3.92
.ANG..
Since they assume a face-centered configuration, the magnitudes
.sqroot.2a and .sqroot.2b may well be regarded as representing the
basic lattices. In this case, the lattice constants, a and b, are
expressed as 5.32 to 5.54 .ANG..
In contrast, MgO which is a material now in widespread use for
substrates has a lattice constant, a=4.203 .ANG., and a lattice
mismatch ratio as high as to reach a range of 7 to 11%. Thus, it
allows production of an epitaxial film of good quality only with
difficulty.
SrTiO.sub.2 possesses a small lattice mismatch ratio in the range
of 0.4 to 4% and exhibits an excellent lattice-matching property.
The SrTiO.sub.3 single crystal, however, is produced at present
solely by the Bernoulli method. The crystal obtained by this method
exhibits very poor crystallinity and possesses an etch pit density
exceeding 10.sup.4 pits/cm.sup.2. It allows production of an
epitaxial film of high quality only with difficulty. Further,
substrates of an appreciably large size are not procurable.
SUMMARY OF THE INVENTION
This invention, produced in the urge to eliminate the drawbacks of
the prior art described above, aims to provide a novel
single-crystal substrate material capable of readily forming an
epitaxial film of good quality.
Another object of this invention is to provide a method capable of
producing a thin oxide superconducting film excelling in
crystal-linity and possessing high quality.
To accomplish the objects described above in accordance with the
first aspect of the present invention, there is provided a
Sr--La--Ga type oxide single crystal possessing a crystal structure
of the K.sub.2 NiF.sub.4 type and having a composition of
Sr.sub.1-X La.sub.1-Y Ga.sub.1-Z O.sub.4-W (-0.1<X<0.1,
-0.1<Y<0.1, -0.1<Z<0.1, -0.4<W<0.4), and
befitting a substrate material.
Then, according to the second aspect of the present invention,
there is provided a method for the production of a thin oxide
superconduting film,characterized by using as a substrate the
aforementioned SrLaGaO.sub.4 single crystal which is a high-melting
oxide possessing a lattice constant closely approximating that of a
pertinent oxide superconductor and effecting epitaxial growth of a
thin oxide superconducting film on the substrate.
As described above, the SrLaGaO.sub.4 single crystal according with
the present invention is an oxide insulator which possesses a
lattice constant falling in the range in which the lattice
constants of oxide superconductors possessing high critical
superconducting temperatures fall, also possesses a crystal
structure closely approximating those of the oxide superconductors,
and exhibits a highly satisfactory lattice-matching property. In
accordance with this invention, therefore, a thin superconducting
oxide film possessing high magnitudes of Tc and Jc can be produced
because epitaxial growth of a thin oxide superconducting film can
be attained by using as a substrate therefor the aforementioned
SrLaGaO.sub.4 single crystal.
These and other objects, aspects, and advantages of the present
invention will become apparent to persons skilled in the art as the
disclosure is made in the following description of preferred
embodiments cited as examples conforming to the principle of this
invention.
DETAILED DESCRIPTION OF THE INVENTION
The high-temperature oxide superconductors reported to date have
lattice constants, a and b, in the range of 3.76 to 3.92 .ANG. as
already mentioned. The present inventor has continued a study in
search of a substrate grade single crystal possessing a lattice
constant of 3.85 .ANG., a mean value of the range just mentioned.
He has consequently found that in the various crystals possessing
the lattice constant, a=3.85 .ANG., SrLaGaO.sub.4 can constitute
itself an outstanding material for a substrate.
It has been reported by G. Blasse (J. Inorg. Nucl. Chem., (1965),
Vol. 27, pp. 2683-2684) that SrLaGaO.sub.4 assumes a K.sub.2
NiF.sub.4 type structure and possesses a lattice constant, a=3.84
.ANG.. What was produced by G. Blasse was a sintered mass and not a
single crystal. K. Aso produced a sintered mass of SrKaGaO.sub.4
and tested it to determine its lattice constant (J. Phys. Soc.
Jpn., (1978), Vol. 44, No. 4, pp. 1083 to 1090). These two reports
are the only information published so far concerning the compound
SrLaGaO.sub.4. The products covered thereby are invariably sintered
masses and not single crystals. For use as a substrate intended for
the production of an epitaxially grown film, a sintered mass which
is a polycrystal is undesirable. The substrate should be a single
crystal. Neither the feasibility of the production of a single
crystal of SrLaGaO.sub.4 nor the melting point of this oxide is
reported anywhere in the literature published to date.
The present inventor, therefore, conducted a study on the
transformation of SrLaGaO.sub.4 into a single crystal and tried the
production of the single crystal by the crucible cooling method. As
regards synthesis of a raw material, a sintered mass of
SrLaGaO.sub.4 was obtained by mixing SrCO.sub.3, La.sub.2 O.sub.3,
and Ga.sub.2 O.sub.3 in a molar ratio of 2:1:1 calcining the
resultant mixed powder at 1,200.degree. C. pulverizing and then
pressing the calcined powder, and sintering the pressed mass of the
powder. This sintered mass was placed in a platinum crucible,
heated to a level exceeding the melting point, and then gradually
cooled at a rate of 1.degree. C./min. As the result, a large planar
single crystal measuring the square of 5 to 15 mm and having the
plate surface as a C plane was easily obtained. This single crystal
had a melting point of about 1,520.degree. C., indicating that it
was a sufficiently high-melting crystal.
The composition capable of forming a single crystal was confirmed
to be in the following range: Sr.sub.1-X La.sub.1-Y Ga.sub.1-Z
O.sub.4-W' wherein -0.1<X<0.1, -0.1<Y<0.1,
-0.1<Z<0.1, and -0.4<W<0.4.
The inventor tried the production of the single crystal by the
Czokralsky method. Specifically, a single crystal of a [001] axis
measuring 25 mm in diameter and 100 mm in length was obtained in a
1 vol % O.sub.2 --N.sub.2 atmosphere at a pulling speed in the
range of 2 to 6 mm/hr and a crystal revolution umber in the range
of 10 to 60 rpm.
The single crystal of SrLaGaO.sub.4 can be produced by the crucible
cooling method and the Czokralski method as described above.
Otherwise, it may be produced by the zone melting method and the
Bridgman method as well.
The single crystal of SrLaGaO.sub.4 has a lattice constant, a=3.84
.ANG.. Since it is a face-centered tetragonal crystal, it may be
regarded as having .sqroot.2a as a basic lattice and, therefore,
possessing a lattice constant, a=5.43 .ANG.. Since the lattice
constants, a and b, of the oxide superconductors fall in the range
of 3.76 to 3.92 .ANG. or in the range of 5.32 to 5.54 .ANG. as
described above, their lattice mismatch ratios relative to the
SrLaGaO.sub.4 are invariably so small as to fall within .+-.2%. The
single crystal possesses a crystal structure closely approximating
those of the oxide superconductors and exhibits an outstanding
lattice-matching property.
As the result, by using a substrate of SrLaGaO.sub.4 and
superposing thereon by epitaxial growth a thin oxide
superconducting film by the spattering method, the vacuum
evaporation method, etc., a thin oxide superconducting film
possessing highly satisfactory crystal-linity can be easily
obtained.
EXAMPLES
Now, the present invention will be described more specifically
below with reference to working examples. Of course, this invention
is not limited to these examples.
EXAMPLE 1
This example concerns production of a Sr--La--Ga type oxide single
crystal.
To obtain a single crystal of the composition of SrLaGaO.sub.4,
678.3 g of La.sub.2 O.sub.3 (purity 99.99%), 614.7 g of SrCO.sub.3
(purity 99.999%), and 390.2 g of Ga.sub.2 O.sub.3 (purity 99.999%)
were mixed and the resultant mixture was calcined at 1,200.degree.
C. for decarbonation, then pulverized, and press molded. A sintered
mass of SrLaGaO.sub.4 weighing approximately 1,500 g was obtained
by sintering the molded mass in the open air at 1,300.degree.
C.
This sintered mass was placed in an iridium crucible measuring 800
mm in outside diameter, 80 mm in height, and 2 mm in wall thickness
and then liquefied by high-frequency heating therein. A nitrogen
atmosphere containing 0.5 to 2% of oxygen was used to envelope the
site of the heat treatment. Since vaporization of a small portion
of the gallium oxide occurred under a nitrogen atmosphere
containing no oxygen, the addition of oxygen in the amount
indicated above was found to be desirable.
After the content of the crucible was fused, a seed crystal was
immersed in the melt and processed in accordance with the
Czokralski method to induce growth of a single crystal of
SrLaGaO.sub.4.
Initially, a [100] single crystal of SrTiO.sub.3 was used as the
seed crystal. After the single crystal of SrLaGaO.sub.4 was
obtained, the single crystal in the [001] orientation of the
produced crystal was used as a seed crystal. The crystal was pulled
at a pulling rate of 5 mm/hr and a crystal rotating speed of 30
rpm. Under these conditions, a [001] axis single crystal measuring
25 mm in diameter and 100 mm in length was obtained.
It was confirmed that a single crystal of high quality could be
produced so long as the composition was in the following range:
Sr.sub.1-X La.sub.1-Y Ga.sub.1-Z O.sub.4-W' wherein
-0.1<X<0.1, -0.1<Y<0.1, -0.1<Z<0.1, and -0.4<W
0.4.
EXAMPLE 2
A (001) plane single crystal of SrLaGaO.sub.4 and a (100) plane
single crystal of SrTiO.sub.3 for comparison were used. On these
substrates, a thin oxide film was superposed in a thickness of
1,000 .ANG. by the RF magnetron spattring using a target of
YBa.sub.2 Cu.sub.3 O.sub.7-.delta. under an atmosphere of
Ar/O.sub.2 (mixing ratio 1:1). The conditions for the spattering
were 10 Pa of gas pressure, 300 W of electric power, and
600.degree. C. of substrate temperature. After the superposition,
the thin oxide films produced were annealed under an O.sub.2
atmosphere at 900.degree. C. for 1 hour.
By the four-terminal method, the produced thin films were tested
for zero resistance temperature Tco and critical superconducting
current Jc at 77.degree. K. The results are shown in Table 1.
TABLE 1 ______________________________________ Tco and Jc of
YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Material of substrate Tco (K.)
Jc (A/cm.sup.2) at 77.degree. K.
______________________________________ Conventional method
(SrTiO.sub.3) 79 0.5 .times. 10.sup.4 This invention
(SrLaGaO.sub.4) 84 1 .times. 10.sup.5
______________________________________
It is clearly noted from the results given above that the product
obtained by using the single crystal of SrLaGaO.sub.4 as a
substrate excelled both in Tco and Jc the product obtained by using
the SrTiO.sub.3 as a substrate. This fact may be logically
explained by a postulate that the film produced with the
SrLaGaO.sub.4 substrate excelled crystallinity and uniformity and,
therefore, enjoyed improvement in Tco and Jc.
When the surfaces of the produced thin films were examined by
reflection high-speed electron diffraction (RHEED) as to
crystal-linity, it was found that the thin film formed on the
SrLaGaO.sub.4 substrate showed a sharp spot-like diffraction
pattern representing a (001) orientation, indicating that it was a
(001) single crystal and produced epitaxial growth.
When thin oxide films were formed each on the (001) plane single
crystal as a substrate under the same conditions as described
above, excepting LnBa.sub.2 Cu.sub.3 O.sub.7-.delta. (Ln=Yb, Er,
Ho, or Gd) was used as a target material, it was found that the
produced films invariably produced epitaxial growth.
EXAMPLE 3
A (001) plane single crystal of SrLaGaO.sub.4 and a (100) plane
single crystal of SrTiO.sub.3 for comparison were used. On these
substrates, a thin oxide film was superposed in a thickness of
1,000 .ANG. by the RF magnetron spattering using a target of
Bi.sub.4 Sr.sub.2 Ca.sub.3 Cu.sub.4 O under an atmosphere of
Ar/O.sub.2 (mixing ratio of 2:1). The conditions for the spattering
were 5 Pa of gas pressure, 200 W of electric power, and 600.degree.
C. of substrate temperature. After the superposition, the thin
oxide films produced were annealed under an O.sub.2 atmosphere at
900.degree. C. for 1 hour.
By the four-terminal method, the produced thin films were tested
for zero resistance temperature Tco and critical superconducting
current Jc at 77.degree. C. The results are shown in Table 2.
TABLE 2 ______________________________________ Tco and Jc of thin
BiSrCaCuO film Material of substrate Tco (K.) Jc (A/cm.sup.2) at
77.degree. K. ______________________________________ Conventional
method (SrTiO.sub.3) 90 1.5 .times. 10.sup.4 This invention
(SrLaGaO.sub.4) 105 2 .times. 10.sup.6
______________________________________
It is clearly noted from the results given above that on the
SrLaGaO.sub.4 substrate, better Tco and Jc were obtained presumably
because the film formed thereon excelled in both crystallinity and
uniformity.
EXAMPLE 4
A (001) plane single crystal of SrLaGaO.sub.4 and a (100) plane
single crystal of SrTiO.sub.3 for comparison were used. On these
substrates, a thin oxide film was superposed in a thickness of
1,000 .ANG. by the RF magnetron spattering using a target of
Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.X under an atmosphere of
Ar/O.sub.2 (mixing ratio 1:1). The conditions for the spattering
were 10 Pa of gas pressure, 80 W of electric power, and 600.degree.
C. of substrate temperature. After the superposition, the thin
oxide films produced were wrapped in gold foil and annealed under
an O.sub.2 atmosphere at 905.degree. C. for 10 minutes.
By the four-terminal method, the produced films were tested for Tco
and Jc at 77.degree. K. The results are shown in Table 3.
TABLE 3 ______________________________________ Tco and Jc of thin
TlBaCaCuO film Material of substrate Tco (K.) Jc (A/cm.sup.2) at
77.degree. K. ______________________________________ Conventional
method (SrTiO.sub.3) 92 0.5 .times. 10.sup.4 This invention
(SrLaGaO.sub.4) 107 5 .times. 10.sup.5
______________________________________
It is clearly noted from the results given above that, on the
SrLaGaO.sub.4 substrate, better Tco and Jc were obtained presumably
because the film formed thereon excelled in crsytallinity and
uniformity.
* * * * *